Tony Blair yesterday put nuclear power at the heart of a national debate on how to solve Britain's looming energy crisis, promising a decision on whether the government would give the green light to a new generation of nuclear plants by early next summer.

The prime minister warned that the closure of ageing coal and nuclear power plants over the next 15 years would cut Britain's electricity generating capacity by almost a third, with renewable sources able to make up only part of the shortfall.

In response, the government is commissioning a review of energy policy to look at how to plug the gap. Mr Blair said it would include "the issue of whether we facilitate the development of a new generation of nuclear power stations".

Yesterday's decision to take a hard look at nuclear energy marks a shift in the government's position from that outlined in a white paper on energy policy two years ago. Though it did not then rule out building new nuclear capacity it said: "Current economics make it [nuclear power] an unattractive option for new, carbon-free generating capacity and there are also important issues of nuclear waste to be resolved."

However, the government has since come under pressure to rethink its energy policy amid concerns about global warming, security of the UK's energy supply as North Sea output declines and rising oil and gas prices. As Britain increasingly imports more of its energy supplies, the country will have to compete for supplies on world markets already having to cope with growing demand from fast-growing economies such as China and India.

Mr Blair told delegates at the CBI's annual conference that the energy issue was back on the global agenda "with a vengeance". He added: "Round the world you can sense a feverish rethinking. Energy prices have risen. Climate change is producing a sense of urgency."

Malcolm Wicks, the energy minister, who will head the review, said the aim was to ensure "clean, reliable, affordable energy supplies for the long term". Circumstances had changed since 2003, he said, which made this the right moment to assess where Britain was in relation to the goals set by the white paper touching on issues such as progress on ensuring that 10% of electricity came from renewables by 2010.

He dismissed any suggestion that decisions had already been taken on the critical issue of new atomic power stations. "It is not a nuclear review. There is no foregone conclusion on nuclear or anything else. It certainly is not a case of nuclear versus renewables," he said.

Mr Wicks admitted that the current gas price spike, which has triggered fears that there could be power shortages if not blackouts for manufacturing in a bad winter, had affected government thinking. In a hint that there could be greater government involvement in the future, he said: "We are going to draw on lessons because of this difficult period we have been through. In terms of an energy review it makes sense to see if we are in the right place as regards the role of the state."

Mr Blair was given a foretaste of the opposition the government can expect to a nuclear new-build programme when two Greenpeace protesters climbed into the iron framework holding up the roof of the main hall in the Business Design Centre in Islington, north London, where he was due to announce the review. They dropped small paper slips bearing the message "Nuclear: Wrong answer" and unfurled banners carrying the same words as they perched precariously high above the conference floor for two hours.

The CBI director general, Sir Digby Jones, said he had been told that the two men, Nyls Verhauelt and Huw Williams, would come down only if Greenpeace was allowed a 10-minute address to the conference ahead of the prime minister.

A compromise offering them the chance to make a statement after Mr Blair had spoken was refused, so the CBI switched the venue of the speech to another hall in the centre and promised a review of its security measures. Greenpeace said the two men had later been arrested on suspicion of aggravated trespass.

Most of the cabinet are said to be open-minded about the nuclear option, although a group including Patricia Hewitt and Peter Hain remain to be convinced. A number of Labour backbenchers remain opposed to expanding nuclear power.

The Tory industry spokesman, David Willetts, criticised the government for taking so long to set up a review.